Protection of the experiment set-up at low temperatures against EMI (pickup)

Transcription

1 Protection of the experiment set-up at low temperatures against EMI (pickup) Jean-Luc Mocellin Chichiliane le 20 septembre

2 Characteristics of measurements at very low temperatures Very reduced heat contribution Very low signal level (pa, fw ) Measuring at the same level of the amplifiers noise energies : - Amplifiers FET : J - Amplifiers S.Q.U.I.D.* : J *Superconducting Quantum Interfernece Device 2

3 Example: measuring resistor 1W Power of measuring [W] 1mW Basic ohmmeters 1µW 1nW pW 1fW Temperature K 1aW 1mK 10mK 100mK 1K 10K 100K 3

4 Intensity of interferences Sensitivity devices Solutions to apply? Precautions to apply Spectrum of interferences Frequency Band to measure 4

5 Interferences Origin 1. Electromagnetic (Mainly) Switching power supplies, mobile, wireless devices, GPS, fluorescent lamps, Wi-Fi (Rise in frequency of the interference source) 2. vibrations - Pumps, ventilators, vibrations of the ground earth - Acoustics waves 5

6 Experiment Set-up Cryostat W 6

7 Consequences High protection against the electromagnetic pickup Protection on all the frequency band, from Hz to GHz Protection against vibrations 7

8 1. Electromagnetic Interference (Pickup) 1.1 Transmission modes 1.2 Protection against interferences 8

9 1.1 Transmission modes By conduction - 230V~ cable - Cable connected to computers - Connections with various grounds (230V~, water cooling circuit ) - «Loops of grounds» By radiation - R.F. radiation - Electric or magnetic fields: leakage field of electrical transformer or switching power supply, motors, PC Screen etc. 1. Transmission mode 9

10 1.1.1 Transmission by conduction Often by various grounds, 230V~, main power supply or digital links i i Measuring device i Cryostat Drain tube return Helium i 230V~ Ground 1. Transmission mode W 10

11 Example 1 : Transmission by main transformer Capacitor about 100pF Leakage current ~ 3µA (30% of current source) i 230V ~ Current Source 10µA Load i 1. Transmission mode 11

12 Example 2 : Switching power supply Added capacitor ~ 1 to 100nF leakage current : 30µA à 3mA! 230V ~ Decoupling Rectification And filter added C Medical power supply : I must be < 100/300µA 1. Transmission mode 12

13 1.1.2 Transmission by radiation E/H Measuring device E i E Cryostat The set-up experiment constitutes an antenna for E.M. waves 1. Transmission mode W 13

14 At low frequency : Types of coupling Wavelength >> dimensions of the set-up, Can be considered separately: Coupling with the magnetic field(inductive) Coupling with the electric field (capacitive) At high frequency : Coupling with both components of fields 1. Transmission mode 14

15 Coupling with the magnetic field and inductive coupling Loop S H The Lenz s law V V = µ S dh/dt Sources of interference fields LF : Electric transformer, Motors, networks cables 10 à 500KHz : Switching power supply > 500KHz : RF transmitter, electric arcs Order of magnitude : From 1Hz to 1MHz : 10nV/cm 2 to 1µV/cm 2 Leakage field of electric transformer at 5cm : 1 to 100µV/cm 2 1. Transmission mode 15

16 Coupling with the electric field and capacitive coupling S E V i C Z i i = ε S de/dt i = C dv/dt Sources of interfering fields 10 to 500 KHz : 230V~ cable, switching power supply > 500KHz : Radiation of the transmitters Order of magnitude on a single conductor from 1Hz to 1 MHz : 10nA to 1µA/cm 1. Transmission mode 16

17 1.2 Protection against the E.M. interference Interference by conduction Interference by Radiated or induced by electric and magnetic fields 17

18 1.2.1 Interference by conduction Interposition of a Barrier Main supply : Shielded transformer, filter Isolation transformer Power line Filter 220V~ Measuring device 0 à 30KHz > 30KHz Digital Transmission : Optocoupler, impulse transformer, optical fibres, WiFi 18

19 1.2.2 Radiated interference or induced by electric and magnetic field Faraday cage Efficient if skin deepth (skin effect) is higher than: µo : perméabilité magnétique du vide (4p.10-7) µr : perméabilité magnétique relative du conducteur (on prendra 1 pour le cuivre) f : fréquence en Hz ρ : résistivité du conducteur en W.m ( W.m pour le cuivre) E = f ρ µ rµ 0 Examples : Copper, at 1 MHz : E = 66 µm Copper, at 50 Hz : E = 9 mm Good attenuation for frequencies higher > 100KHz 19

20 Inconvenience of the Faraday cage Inefficient for continous magnetic field and low frequencies E H > 100 KHz H < 1 KHz Faraday cage Measuring device E The device produces interfence must be placed outside Not easy to made Expensive 20

21 Solution «Faraday cage» but reduced to a whole of screens and shieldings surrounding the sensitive circuits Measuring device Cryostat 21

22 Weaknesses of this Faraday screen room 1 - Transparency to the low frequency magnetic fields 2 - Many connecting cable 3 - Connectors 4 - Boxes and cases 22

23 1- Transparency to the low frequency magnetic fields How to protect itself? Loop surfaces reduction Magnetic shieldings 23

24 1.1 - Loop surfaces reduction Connection by twisted pair Connection by coax P.C.B. : A reference wire walks olong with the signal wire (the ground is not a perfect equipotential at low frequency) 24

25 1.2 - Magnetic shieldings Case or basic tube steel Small attenuation : factor 2 to 5 (at 50 Hz) Case : Mumetal, Co-Netic, MµShield Attenuation 300 to 1000 Rolling up of amorphous metal ribbon Attenuation 300 to 1000 No annealing Ferromagnetic & copper alternate layers 25

26 Calculation of a cylindrical shielding H Attenuation : A = µ*e / D Thickness : e > 1,25*D*H / B e D µ : Permeability of material D : Diameter of the tube e : Thickness of the tube H : External magnetic field B : Induction in material 26

27 2 - Connecting cables E/H Measuring Device E i i E Cryostat Main Ground 27

28 2.1 - Interference by the currents in the shieldings 28

29 Example : Shielding resistance of the coax : 17 mω Induced voltage : Linear power supply : 50 nv Switching power supply : 50µV 29

30 Remedies Short cable, cable with low impedance transfer Connection by twisted pair Reduce the current in the shieldings Differential amplifier Isolation amplifier 30

31 Impedance transfer of the cables Z 2V = t L * I Must be as low as possible 31

32 Impedance transfert of coax câble for some types of shielding Impedance mo Ohm/m 10 1 Une One tresse Braid 2 Two tresses Braids Copper tube Tube cuivre Frequency MHz 32

33 Connection by twisted pair Residual effect V 0 si Z1 Z2 33

34 Good compensation with a differential amplifier 34

35 Reduction of the current in the shieldings Inductance of common mode 35

36 Double grounding circuit 36

37 Some types of inductances of common mode : From 100KHz to 10MHz Inductance on tore ferrite of 0,1 to 100mH wound «two wire in hand» Twisted pair 37

38 Broad band signal: Coax wound on tore Nature of tore depending on frequency band - 10KHz to 1MHz : Alloy high permeability - 100KHz to 100 MHz : Ferrite 38

39 Beyond 30MHz : one or few tubes of ferrite Double ground circuit Equivalent diagram 39

40 Flotting input mode Differential amplifier With inductance of common mode to compensate the differential amplifier rejection fall 40

41 2.2 - Electric field interference Covering rate of the braid : 70 à 95% Residual coupling ~ 0,1pF/m Use shielded cables by two braids or by braid plus aluminized sheet protect the cable by a conductrice sheath 41

42 Basic rules for chosing the cables µv level sensitivity, use : Coax cable, (no basic shielded cable) Shielded cable by braid, (covering rate > 95%) 10nV level sensitivity, use : Short coax cable (< 20cm) Double braid cable or braid + aluminized sheet Twisted pairs Shielded + aluminized sheet nv level sensitivity, use : Rigid and short coax cable Triaxial cable Case connected directly on the cryostat 42

43 Anything wrong? Signal Source Differential Amplifier Coax cable 43

44 Signal Source H ~ Differential Amplifier V Induced voltage in the loop formed between the coax 44

45 Signal Source H ~ Differential Amplifier i i The current i induced in the loop made by the shields are opposed to the flux variation These currents reduce the interference voltage with an increasing effectiveness folowing the frequency (starting at ~ 10KHz) 45

46 3 - Connectors Braid of the shield carefully connected to the case of the connector Shielded cable Braid of the shield turned over choose connectors which ensure a continuity of the shielding on

47 4 - Boxes & cases Closed boxe Good continuity between walls Short contact points : Lenght between 2 points << λ Seal HF Metal knitting seal Conducting rubber 47

48 Problem of the high frequencies The effectiveness of shieldings cables decreases With the frequency increase. At the very low temperature (< 1 K), power entering the cryostat is too high. P<10-15 W 48

49 Installation of crossing filter Crossing filter, Base plate filter P<10-15 W 2. Protection against the electromagnetic interference 49

50 Crossing filter Type : C, RC, LC, RLC, made in L, T or PI Cutoff frequency < 1MHz (if possible) Crossing capacitor Inductance at High resonance frequency 50

51 2. Interference due to vibrations Ground vibration Acoustic noises Both Generate : Release of heat electric interference signals: microphonism 2. Vibrations 51

52 Protection against ground vibration insert between the cryostat and the ground a low-pass mechanical filter : Antivibratory table Heavy support: concrete, case of sand resting on elastic feet: springs, blocks of rubber, insulators with air cushion 2. Protection against vibration interference 52

53 Protection against acoustic noise Generally negligible effect If necessary, to lock up the electronic in a made box of dense walls and to cover them with materials absorbing (polyurethane foam ) 2. Protection against vibration interference 53

54 Microphonism Vibration of a cable in a magnetic field Vibration of a cable under power triboelectric Effect Microphonism of transformers Piezoelectricity 2. Protection against vibration interference 54

55 Vibration of a cable in a magnetic field Very important effect in the field of a coil transmit the signal per twisted pair or coax To note: The vibrations can be generated by a AC current circulating in wire Twist the wire back and forth of the current 2. Protection against vibration interference 55

56 Vibration of cable under power c I V Z I = V*dC/dt Assign especially the circuit to high impedance 2. Protection against vibration interference 56

57 Triboelectric effect Observed mainly in the cables Electric charge generated by the friction of dielectric on the shielding Value : from pa to na! Assign especially the circuit to high impedance Use cable treated anti-signal (dielectric covered with a conducting layer) 2. Protection against vibration interference 57

58 Microphonism into transformers Transformers of impedance adaptation to the amplifiers input Field of the low frequencies (1Hz à 100KHz Dependent on the magnetic properties of the core 2. Protection against vibration interference 58

59 Piezoelectricity The interference effects occur mainly in certain ceramic capacitor (dielectric with high permittivity) 2. Protection against vibration interference 59

60 B. P. 16 Avenue des Martyrs GRENOBLE CEDEX 09 Jean-Louis Bret & Jean-Luc Mocellin 60